This invention relates to ether production employing a catalytic distillation technique. In particular, it relates to synthesis of unsymmetrical ethers, such as methyl t-butyl ether (MTBE) or t-amyl alkyl ether by reaction of a lower alkanol with a tertiary alkene in a multizone fixed bed reactor, such as a catalytic distillation ("catstill") tower.
There is a need for an efficient catalytic process to manufacture ethers from the reaction of light olefins with lower alkanols augmenting the supply of high octane blending stocks for gasoline. Relatively low molecular weight ethers such as methyl-t-butyl ether (MTBE) and t-amyl methyl ether (TAME) are in the gasoline boiling range and are known to have a high blending octane number. The present invention provides an improved process for the catalytic reaction of isoalkenes (branched olefins) with lower alkanols (e.g. C.sub.1 -C.sub.4 primary aliphatic alcohols) to provide ether(s). More particularly, the invention relates to a process for the reaction of one or more tertiary olefins such as isobutene, isopentene(s), isohexene(s), etc., or mixtures thereof, with one or more lower alkanols, e.g., methanol, ethanol, n-propanol, isopropanol, etc., or mixtures thereof, to provide one or more ethers employing the acidic form of a particular synthetic porous acidic catalyst material as catalyst. The product ethers are useful as high octane blending stocks for gasoline.
While the use of catstill units has contributed greatly to industrial application of catalytic etherification process employing volatile alkanol and isoalkene reactants, the operating conditions of catstill units require a substantial vertical temperature gradient in the catalytic reaction zone. This variable temperature has a significant effect on reaction kinetics. Conventional etherification processes use as catalyst a macroreticular cation exchange resin in the hydrogen form, such as "Amberlyst 15". A resin catalyst gives a range of about 65.degree. to 90.degree. C. In existing conventional catstill designs, an upper rectifying section utilizes trays or packing rather than catalyst solids due to decrease in catalyst activity at lower operating temperature, i.e., below about 65.degree. C. (150.degree. F.). Since etherification reactions have an equilibrium which favors ether production at lower temperature, it is desirable to employ catalysts and reaction conditions which permit low temperature operation.
It has been discovered that certain acid metallosilicates have catalytic activity for the desired etherification reaction at temperature not greater than 65.degree. C., thus permitting the advantageous catalytic reactions to take place in an upper rectification zone of a multizone catalytic distillation unit. Early catstill units for MTBE production employed sulfonic acid resin solid catalysts in a fixed packed bed supported in a vertical debutanizer tower.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material.
U.S. Pat. No. 4,605,787 (Chu and Kuehl) discloses the preparation of alkyl tert-alkyl ethers such as MTBE and TAME by the reaction of a primary alcohol with an olefin having a double bond on a tertiary carbon atom employing as catalyst an acidic zeolite having a constraint index of from about 1 to 12, e.g., zeolite ZSM-5, 11, 12, 23, dealuminized zeolite Y and rare earth-exchanged zeolite Y. European Patent Application 55,045 describes a process for reacting an olefin and an alcohol to provide an ether, e.g., isobutene and methanol to provide MTBE, in the presence of an acidic zeolite such as zeolite Beta, zeolites ZSM-5, -8, -11, -12, -23, -35, -43 and -48, and others, as catalyst. U.S. Pat. No. 4,962,239 describes the etherification of olefins with an alcohol over a catalyst comprising a zeolite designated MCM-22. Similar intermediate pore materials may be employed, such as MCM-36, described in U.S. application Ser. No. 07/811,360, filed Dec. 20, 1991, incorporated herein by reference.
Recent efforts in the field of etherification reactions have focused on the use of acid medium-pore zeolite catalyst for highly selective conversion of isoolefin and alcohol starting materials. In U.S. Pat. No. 5,015,782 (Harandi and Owen) zeolitic metallosilicate catalysts such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-50 and zeolite Beta have been reported as desirable high temperature catalysts for MTBE and TAME production. It has been thought that, due to lower acidity as compared to resin catalysts, the zeolites need to be employed at higher reaction temperature to achieve the desired conversion rates. These solid acid catalyst particles are thermally stable, are less sensitive to methanol-to-isobutene ratio, give no acid effluent, and are easily and quickly regenerated.